1. Field of the Invention
This invention relates to a method for preparing a photoconductive composition containing a microparticulate pigment or dye, more particularly to an electrophotographic photosensitive member which can improve electrophotographic characteristics and to a method for preparing an organic photoconductive composition.
2. Description of the Prior Art
In the prior art, as the electrophotographic photosensitive member comprising an inorganic photoconductive material, there have widely been used those employing selenium, cadmium sulfide, zinc oxide, etc.
On the other hand, as the electrophotographic photosensitive member comprising an organic photoconductive material, there have been known those employing photoconductive polymers, typically poly-N-vinylcarbazole, or low molecular weight organic photoconductive materials such as 2,5-bis(p-diethylaminophenyl)-1,3,4-oxadiazole, and further combinations of such organic photoconductive materials with various kinds of dyes and pigments.
Electrophotographic photosensitive members employing organic photoconductive materials have the advantages of good film forming property, very high productivity because they can be produced by coating and low cost of the photosensitive members produced. Also, there is an additional advantage that color sensitivity can be controlled as desired by choice of the sensitizers employed such as dyes or pigments, and therefore these photosensitive members have widely been investigated. Particularly, in recent years, by development of a function separation type photosensitive member, in which a charge generation layer of an organic photosensitive pigment and a so-called charge transport layer comprising a photoconductive polymer or a low molecular weight organic photoconductive material as mentioned above are laminated, sensitivity and durability of the organic electrophotographic photosensitive member, which have been deemed to be the drawbacks in the prior art, are improved and they are becoming utilized for practical application. Further, various kinds of compounds and pigments sensitive to the function separation type photosensitive member have been found.
On the other hand, in such a function separation type photosensitive member, which is constituted of at least two layers of a charge generation layer and a charge transport layer, the charge carriers generated by light absorption by the charge generation layer are injected into the charge transport layer to have the surface charges of the photosensitive member disappeared, thereby forming an electrostatic contrast. This type of photosensitive member will be affected in its sensitivity by the particle size of the charge generating material contained in the charge generation layer, and a charge generation material with a particle size of about 1.mu. or less, desirably 0.5.mu. or less, is known to be preferable for sensitivity. For this reason, in the preparation method of the prior art, after purification of the pigment or dye obtained by synthesis reaction, the powdery product obtained once by drying by heating is subjected to micropulverization together with a binder to about 1.mu. or less, desirably 0.5.mu. or less, by means of a sand mill, ball mill or an attritor.
However, the particles obtained according to such a method of the prior art will be changed in state of the particle sizes according to the changes in temperature or humidity during the preparation steps. Particularly, when heated to a temperature of 50.degree. C. or higher, coarse particles will be liable to be formed, and it is necessary to effect sufficiently pulverization of the dye. Practically, however, there is involved a technically difficult problem in obtaining microparticulate composition in which the number of coarse particles is sufficiently reduced by way of the pulverizing step. Besides, since the state of particles agglomerated will be changed depending on the preparation environment, it is not possible to apply micropulverization treatment under constant conditions, thus creating a difficulty in manufacturing steps. Also, since coarse particles are contained in a large amount, the photosensitivity suffers from not only lowering in number of carriers generated accompanied with lowering in shielding force, but also lowering in carrier mobility caused by increase of the void volume by the coarse particles. Further, due to great unevenness on the surface of the charge generation layer, the efficiency of carriers injected into the charge transport layer will be lowered. Thus, a large number of drawbacks in sensitivity are involved, and yet there is an additional disadvantage that the potential stability during repeated uses is lowered.